FIG. 6 shows the main portion of a solid polymer fuel cell, in which a membrane electrode assembly (MEA) 1 is sandwiched between separators (not shown). A number of MEAs are disposed in the fuel cell. The MEA 1 comprises a hydrogen electrode-side catalyst layer 11a formed on one side of an electrolyte membrane 10 that comprises ion-exchange resin and an air electrode-side catalyst layer 11b formed on the other side thereof. In general, a catalyst layer of a fuel cell electrode has a structure in which a carbon carrier is allowed to support a noble metal such as platinum and ion-exchange resin covers the surface of such carbon carrier having a catalyst supported thereon. Such catalyst layer is required to have functions of gaseous diffusibility, electronic conductivity, and ionic conductivity, in addition to allowing catalytic reactions to occur therein.
A catalyst layer is formed in one of the following manners: a catalyst ink comprising a carbon carrier having a catalyst supported thereon, a solvent, and ion-exchange resin (electrolyte) is prepared to be applied on an electrolyte membrane by spraying or by an applicator using a doctor blade technique followed by drying; or the catalyst ink is applied on a substrate such as a PTFE substrate or a PET substrate to be thermally transferred to an electrolyte membrane under pressure by hot pressing. Further, gas diffusion layers 12a and 12b are laminated on the catalyst layers 11a and 11b, respectively. Via channels formed on the separators, hydrogen to serve as fuel gas, and in general, air to serve as oxidation gas are supplied to the hydrogen electrode-side catalyst layer 11a and the air electrode-side catalyst layer 11b, respectively.
An improved level of gaseous diffusibility in a catalyst layer is advantageous for the promotion of catalytic reactions in a catalyst layer. Thus, it is common practice to design a catalyst layer having an improved pore volume. In view of this, JP Patent publication (Kokai) No. 8-138715 A discloses a technique for forming a porous electrode catalyst layer with a high porosity by allowing electrode catalyst salts and polymer particles to adsorb together on an electrolyte membrane by dispersion plating involved in chemical plating, followed by removal of the polymer particles using an acidic solution.